1. Field of the Invention
The present invention relates to apparatus and methods for measuring the baseline vectors between a pair of points, such as survey marks, on earth by radio interferometry from information contained within the L1 and L2 transmissions from satellite transmitters of the G.P.S. (Global Positioning System) constellation. More particularly, this invention pertains to a digital system for use in determining the signal phase of satellite transmissions that does not require a priori knowledge of the coding modulation of the satellite transmissions.
2. Description of the Prior Art
The use of pseudo random bit sequences to modulate free space signals has acquired significance in the communication arts with many important applications, both military and civilian. By utilizing such a sequence, often referred to as a pseudo-noise code (PN-code), one may modulate a sinusoidal carrier in such a way as to create a signal of the direct sequence spread spectrum type. In such a signal, more bandwidth is occupied than is required for information transmission. As such, one may attain a number of advantageous characteristics including (1) hiding of signal, (2) inherent anti-jam performance resulting from the spreading of signal power, (3) transmission of ranging information, (4) lessened sensitivity to signal path anomalies and (5) code division multiple access (CDMA) allowing the transmission of many signals in the same frequency band without interference.
The above-described characteristics of direct sequence spread spectrum signal transmissions have proven advantageous in battlefield environments, oil exploration and satellite-aided radio navigation. A prominent application is found in the Global Positioning System ("GPS"), a world-wide network of earth satellite-transmitters that will allow users to fix or determine their locations with respect to known satellite locations. Each of such satellites will transmit both an L1 signal at a nominal (suppressed) carrier frequency of 1575.42 MHz modulated with both an identifying C/A (coarse acquisition) coded pseudo-noise sequence and an identifying P (precision) coded sequence and an L2 signal having a nominal (suppressed) carrier frequency of 1227.60 MHz modulated with an identifying P (precision) coded sequence that is assigned to the satellite. The precise location of a GPS receiver is determined from the known locations of the satellite transmitters by well known techniques of trilateration.
Techniques of determining relative positions of different sites, one with respect to another, from measurements of the phase or the group delay differences between radio signals received simultaneously at those sites are also known in the art and are collectively referred to as techniques of geodesy by radio interferometry. The antennas at the separate sites are considered to form an interferometer, and the relative position vector that extends from one antenna to the other is called the baseline vector of the interferometer. The baseline, or relative-position, vector between two antennas can be determined usually with less uncertainty that the position of either individual antenna can be, because many potential sources of error tend to affect the measurement at both antennas nearly equally, and therefore tend to cancel when differences are taken between the two antennas. The technique of geodesy by microwave radio interferometry is known to provide an unmatched combination of accuracy, speed, and range for the determination of relative-position or interferometer "baseline" vectors. Such a determination may be based upon measurements of either the group-delay difference, or the phase difference, or of both differences between the signals received at the two ends of the baseline vector. Phase measurements are inherently more accurate than group delay measurements, but the interpretation of phase measurements is more complicated due to their intrinsic, integer-cycle ambiguity.
In the prior art, the information for determining baseline vectors from G.P.S. satellite transmissions required knowledge of the coding that modulated the transmission. While the C/A codes will be generally available to the public, P codes will be strictly limited to military applications and, thus, this technique limits the public to L1 transmissions.
A number of "codeless" systems have been developed to alleviate the requirement of having to know the PN-code to obtain information from the satellite signal. Among such systems are those described in U.S. patent Ser. No. 4,667,203 of Charles C. Counselman, III issued May 19, 1987 entitled "Method and System For Determining Position Using Signals From Satellites". That patent discloses a system for determining an earth baseline vector by locating G.P.S. receivers at the end points of such baseline. The system described in that patent relies upon predictions of the doppler shifts of the nominal carriers from multiple satellites, rather than knowledge of the codes modulating such transmissions, to calculate the baseline vector from the L2 satellite transmissions. While the system disclosed in that patent represents an improvement over systems requiring knowledge of the coded modulation of satellite transmissions, the disclosed system is implemented in analog circuitry and therefore employs elements that are inherently more costly, less reliable, more sensitive to temperature change, consume more power and space than digital circuit elements.